Steam boilers comprise a furnace in which fuel is burnt to produce hot gases and then these gases usually pass along a convection pass which contains one or more banks of substantially horizontal tubes. Where the hot gases pass downwardly through the convection pass and water is evaporating in the horizontal tubes, e.g. in an evaporated bank or steaming economiser tube bank, the steam produced separates and forms a blanket across the ceiling of the tube. In such situations it has been found that the downwardly moving hot gases impinge on the tubes in their upper regions where the blankets of steam have collected and those regions are particularly susceptible to excessive heating and internal corrosion.
An object of the invention is at least to reduce the magnitude of this problem.